This is the fūinjutsu that was used to seal Gyūki into Killer B, which takes the form of the kanji for "iron" (鉄, tetsu) on his right shoulder. Not much is known about the seal, besides the fact that it was stated by B to be of lower-quality and weaker than the Four Symbols Seal, which contained Kurama inside of Naruto Uzumaki.[1]

Trivia

In the manga, the seal is depicted as being black. In the anime however, the seal is blue.